How can what you see in this photo help you to figure out what happened in Earth's history? You can see the molten lava and what it looks like when it cools. If you see that type of rock in an outcrop you can assume that it formed from molten lava. This reveals the best tool for understanding Earth history that Earth scientists have. They use what they know about materials and processes in the present to figure out what happened in the past.

Ask a Question – Earth History

The outcrop in the image below is at Checkerboard Mesa in Zion National Park, Utah. It has a very interesting pattern on it. As a geology student you may ask: how did this rock form?

Checkerboard Mesa in Zion National Park, Utah.

If you poke at the rock and analyze its chemistry you will see that it’s made of sand. In fact, the rock formation is called the Navajo sandstone. But knowing that the rock is sandstone doesn’t tell you how it formed. It would be hard to design an experiment to show how this rock formed. But we can make observations now and apply them to this rock that formed long ago.

Uniformitarianism

James Hutton came up with this idea in the late 1700s. The present is the key to the past. He called this the
principle of uniformitarianism
. It is that if we can understand a geological process now and we find evidence of that same process in the past, then we can assume that the process operated the same way in the past.

Let’s go back to that outcrop. What would cause sandstone to have layers that cross each other, a feature called cross-bedding?

Answer a Question – Earth History

In this photo of the Mesquite sand dune in Death Valley National Park, California, we see that wind can cause cross-bedding in sand. Cross-bedding is due to changes in wind direction. There are also ripples caused by the wind waving over the surface of the dune.

The Mesquite sand dune in Death Valley National Park, California.

This doesn’t look exactly like the outcrop of Navajo sandstone, but if you could cut a cross-section into the face of the dune it would look very similar.

Since we can observe wind forming sand dunes with these patterns now, we have a good explanation for how the Navajo sandstone formed. The Navajo sandstone is a rock formed from ancient sand dunes in which wind direction changed from time to time.

This is just one example of how geologists use observations they make today to unravel what happened in Earth’s past. Rocks formed from volcanoes, oceans, rivers, and many other features are deciphered by looking at the geological work those features do today.

Summary

You may need to apply what you know about the present to determine what happened in the past.

The idea that the present is the key to the past was recognized by James Hutton in the late 1700s.

If you see something forming by a process today and then find the end results of that process in the rock record, you can assume that the the process operated the same way in the past.

Practice

Use this resource to answer the questions that follow.

1. How does a geologist use observations about how and where ripple marks are found to understand ripple marks in a rock?